1. Field of the Invention
This invention relates to a machine for metering, mixing and dispensing fluids and, more particularly, to a valve apparatus and electrical control circuitry to control the flow of components of polyurethane, elastomers, or other products into the mixing chamber.
2. Description of the Prior Art
Machines for metering, mixing and dispensing polyurethane are well-known and are used in conjunction with an assembly line for filling a plurality of molds. Each of the molds on the assembly line periodically moves under a dispensing nozzle of a machine to receive a predetermined quantity of polyurethane. A machine operator views the assembly line and controls the machine to dispense the polyurethane into each mold.
Conventional machines for metering, mixing and dispensing polyurethane usually work on a two cycle operation. In one cycle, known as a recirculation cycle, two components which, when combined, form the polyurethane are recirculated about the machine in separate channels. In the second cycle, known as the output or shot cycle, the two components are metered into a mixing chamber where they are mixed to form the polyurethane which is then dispensed into the mold. The operator switches the machine from the recirculation cycle to the shot cycle when a mold moves under the dispensing nozzle.
The conventional machines employ valves which are either fully opened or closed to direct respective components into or away from the mixing chamber. During the recirculation cycle, the recirculation channels are open and the valves closed to the mixing chamber to prevent components from entering the chamber. When a mold is in position to receive the polyurethane, the machine is placed in the shot cycle in which the recirculation channels are closed and the valves turned to the open position to permit the components to enter the mixing chamber.
It is important to meter predetermined quantities of components, mix them and dispense the polyurethane from the mixing chamber within a predetermined period of time. Otherwise, the polyurethane formed in the mixing chamber will foam and clog the chamber as well as the valves. For example, a shot cycle may last about 30 seconds during which time the components will be continuously directed into the mixing chamber, mixed and then dispensed to fill one mold.
Conventionally, the components flow into the mixing chamber through a relatively small aperture of about 1/4". Therefore, a relatively high pressure is needed in the machine to force the required amounts of components into the mixing chamber during the shot cycle. Consequently, conventional machines are constantly operated under steady heat pressure conditions to have available the high pressure needed during the shot cycle. These machines are constantly operating under steady high pressure conditions of about, for example 150-200 psi during both the recirculation cycle and the shot cycle.
A disadvantage with conventional machines is that they continuously operate under steady high pressure conditions though such conditions are required only for the shot cycle. As a result, electric motors driving pumps for the components must be capable of high performance and use large amounts of electric power at all times. Such machines, therefore, waste a great amount of energy since the high pressure needed for filling a mold with polyurethane is required only during the shot cycle. Moreover, even if the assembly line is shut down the machines must continuously recirculate the components forming the polyurethane to avoid clogging of the machines. Since these machines continue to recirculate the components under such high pressure conditions, a large amount of electrical energy is wasted.
Furthermore, because the conventional machines operate under steady high pressure a relatively long period (about 15 seconds) elapses between shot cycles or pours into the molds. This is possibly because the valves controlling the flow direction of the components cannot move quickly between the open and closed positions in a high pressure environment. Therefore, molds must travel along the assembly line at a reduced speed, and the number of molds filled per unit of time is reduced accordingly.
In addition, as indicated above, the valves in conventional machines are in either the fully closed or fully opened position. Consequently, the flow into the mixing chamber cannot be calibrated to vary the amount of components entering the mixing chamber for a given shot cycle. Also, the duration of the shot cycle in such machines cannot be varied without varying the quantity of components metered into the mixing chamber.